1. Field of the Invention
The present invention relates to a method for producing metal wirings which are formed on an insulating substrate for fabricating an active matrix substrate used for a liquid crystal display apparatus or the like.
2. Description of the Related Art
A liquid crystal display apparatus employing an active matrix driving system includes an active matrix substrate which has an array of thin film transistors (hereinafter abbreviated as TFTs) and gate electrode wirings running between the TFTs. Such gate electrode wirings are made of metal which can form a "self oxide film" on the surface by anodic oxidation, thermal oxidation, or the like. After the formation of such an oxide film which functions as an insulating layer, another insulating layer made of SiN.sub.x, SiO.sub.2, or the like is formed. Thus, the gate electrode wirings have a "double-insulation structure".
The gate electrode wirings have gate electrodes extending therefrom, each of which constitutes the TFT together with a source electrode and a drain electrode. The source electrodes extend from source electrode wirings which also run between TFTs in a direction vertical to the gate electrode wirings. When the gate electrode wirings and the gate electrodes extended therefrom have the above double-insulation structure, the insulation property of the gate electrodes from the source electrodes and the drain electrodes is improved, compared with that of the gate electrodes without the oxide film. Examples of metals capable of forming an oxide film include tantalum (Ta), niobium (Nb), titanium (Ti), and aluminum (Al). Especially, Ta is widely used for insulating layers for thin film diodes (hereinafter abbreviated as TFDs) not only for the TFTs since Ta.sub.2 O.sub.5 obtained by anodically or thermally oxidizing Ta exhibits the Poole-Flenkel conduction.
Ta has a lattice structure of two different types: a body-centered cubic lattice structure and a tetragonal cubic lattice structure. The Ta with the body-centered cubic lattice structure is called .alpha.-Ta, and the Ta with the tetragonal cubic lattice structure is called .beta.-Ta. The specific resistance of .beta.-Ta in the form of a thin film is as large as approximately 170-200 .mu..OMEGA..cm, while the specific resistance of .alpha.-Ta in the form of a bulk is as small as approximately 13-15 .mu..OMEGA..cm.
In recent years, a demand for a liquid crystal display apparatus with a wider screen and higher precision has increased. To realize such a liquid crystal display apparatus, gate electrode wirings and source electrode wirings must be made longer and narrower with smaller resistance than conventional ones. To satisfy these requirements, gate electrode wirings are preferably made of a material with small specific resistance such as .alpha.-Ta. In most cases, however, a film obtained by depositing Ta by normal sputtering is a .beta.-Ta film having a high specific resistance. An .alpha.-Ta film can be formed by doping a Ta film with a minute amount of nitrogen at the formation of the Ta film by sputtering. However, due to the presence of this dopant nitrogen, the specific resistance of such an .alpha.-Ta film increases to approximately 60 to 100 .mu..OMEGA..cm, which is too large to be used for the gate electrode wirings. The dependence of the resistivity and temperature coefficient of resistivity of a Ta film on the dose of ions such as Ar.sup.+ and N.sub.2.sup.+ is described in K. H. Goh, et al., "Ion impact chemistry in thin metal films; Argon, oxygen and nitrogen bombardment of tantalum", Ion Implantation in Semiconductors, Plenum Press, pp. 325-333.
It is also known that an .alpha.-Ta film can be formed by depositing Ta on a thin film base such as Nb, Mo, and TaN.sub.x with the body-centered cubic lattice structure, instead of the doping of nitrogen. The deposited Ta is known to become .alpha.-Ta by an influence of the film base. The resultant non-doped .alpha.-Ta film has a specific resistance as small as approximately 20 to 30 .mu..OMEGA..cm, which can be suitably used for the gate electrode wirings.
However, the non-doped .alpha.-Ta has disadvantages as follows: when an oxide film is formed on a surface of the non-doped .alpha.-Ta film, the insulation property of the oxide film is low compared with the case of an .alpha.-Ta film doped with nitrogen.
Further, while a liquid crystal display apparatus requires the symmetry in the voltage-current characteristic with regard to the zero-axis of the voltage, the voltage-current characteristic of Ta.sub.2 O.sub.5 obtained by oxidizing the non-doped .alpha.-Ta without nitrogen is not symmetrical with regard to the zero-axis of the voltage.
For the above reasons, an active matrix substrate including the metal wirings made of non-doped .alpha.-Ta is not suitable for the realization of a large-scale, high-precision liquid crystal display apparatus.
Japanese Laid-Open Patent Publication No. 3-51823 discloses a method of implanting an insulating layer with ions after the formation of the insulating layer so as to ensure a stable voltage-current characteristic for a switching element composed of three-layer structure of metal-insulator-metal for a liquid crystal display apparatus. However, the disclosure does not include a method for reducing the resistance of metal wirings as well as improving the insulation thereof so as to achieve a large-scale, high-precision liquid crystal display apparatus.